An expansion turbine, comprising: a case having an inlet and an outlet for a working fluid; at least one stator (3) installed inside the case; at least one rotor (2) installed inside the case and rotating around a respective revolution axis (X-X); a short-pipe (4) constrained to the case; a mechanical unit (5) installed inside the short-pipe (4). The mechanical unit (5) comprises a bushing (7) and a shaft (6) rotatably installed inside the bushing (7). The shaft (6) is connected to the rotor (2) in removable manner and the whole mechanical unit (5) inclusive of the shaft (6) is extractable from the short-pipe (4) in a unitary form from the side opposite to said rotor (2). The rotor (2) is movable along the axial direction (X-X) between a first configuration, in which the mechanical unit (5) is installed inside the short-pipe (4) and the rotor (2) is spaced from the short-pipe (4), so that the working fluid can revolve it, and a second configuration, in which the mechanical unit (5) is extracted from the short-pipe (4) and the rotor (2) leans agains the short-pipe (4) a static seal (18,19).